Research On Electrodeposition Preparation And Performance Of LiCu Alloy Film For Lithium Battery

Lithium has been known as an attractive anode material for lithium secondarybatteries because of its high theoretical capacity of3860mAh/g. However, serioussafety problems caused by a dendritic growth on the surface during continuouscharge-discharge cycles make metallic lithium far away from commercial applicationsall the time. Aimed at solving this problem, this thesis proposes LiCu alloy filmprepared by electrodeposition as the novel anode material for lithium batteries. Copperin LiCu alloy film serves as a supporting framework to promote the uniformdeposition/resolving of lithium and prevent the expansion/contraction of lithium’svolume during charge-discharge cycles, thus inhibiting the production of dendrite andimproving the safety and stability of metallic lithium anode.Electrodepositions of LiCu alloy from two kinds of ionic liquids include1-butyl-3-methylimidazolium tetrafluoroborate （[BMIm][BF₄]） and1-ethyl-3-methyllimidazolium bis（trifluoromethylsulfonyl）imide （[EMIm][TFSI]） wereboth investigated. Through the contrast research on the microstructure, composition andstability of LiCu alloy films, final deal was made that [EMIm][TFSI] ionic liquid is themore suitable solvent for electrodeposition of LiCu alloy film used as anode for lithiumbatteries. And the optimal electrolyte composition and potentiostatic operatingconditions were determined as below:1.0mol/L LiTFSI+0.066mol/L Cu（PTSA）₂+[EMIm][TFSI]: NMP（in a volume ratio of2:1）, electrodeposition potential is-3V,temperature is298K, depositional time is2h., auxiliary electrode and referenceelectrode in the three-electrode system are platinum foil and platinum wire, respectively.The LiCu alloy anode prepared under the above condition contains74.80at.%freemetal lithium and25.20at.%free metal copper. Its theoretical capacity can reach to943.87mAh/g in lithium batteries. Button cells were assembled using LiCu alloy filmas anode and LiCoO₂/LiFePO₄/Li（NiCoMn）O₂as cathode, respectively. The cyclevoltammetry techniques, charge-discharge tests, SEM and EIS analyses were carried outto study the compatibility of LiCu alloy anode and the above three kinds of cathodematerials. The results showed that LiCu alloy anode had a better compatibility withLiFePO₄. But the cycling performance and charge-discharge efficiency of LiCualloy/LiFePO₄. cell was still not as good as the commercial Li/LiFePO₄cell. Besides,rate performance test indicated that the discharge capacity of LiCu alloy/LiFePO₄celldecreased faster than that of the commercial Li/LiFePO₄cell as the growth of the chargeand discharge rates.Results of the above batteries’ tests indicate that the cycling stability of lithiumbatteries hasn’t been improved by the introduction of LiCu alloy anode prepared by present electrodeposition process. This performance reveals that the action mechanismof LiCu alloy anode in lithium battery is different from expectation. Throughcomparative studies on the surface dynamic performances and infrared spectrumanalyses of LiCu alloy anode and the commercial Li anode after immersion in theelectrolyte for several days, concluded that LiCu alloy thin film electrodes in lithiumbattery for the role of mechanism, the action mechanism of LiCu alloy anode in lithiumbattery was summarized as follows: Due to the smaller metal grains’ size and largerspecific surface area of LiCu alloy film, the higher reactivity with electrolyte is resulted.This leads that the SEI film has complicated component and loose structure, thus makesthe electrode interface impedance and migration resistance of Li+increase rapidly, alsothe cycling stability and charge-discharge efficiency decline eventually.